Process for producing melt blown nonwoven synthetic polymer mat having high tear resistance

ABSTRACT

A HIGHLY TEAR RESISTANT NONWOVEN MAT OF THERMOPLASTIC POLYMER FIBERS IS MADE IN A MELT-BLOWING PROCESS IN WHICH A MOLTEN POLYMERIC RESIN IS EXTRUDED THROUGH A ROW OF DIE OPENINGS INTO A STREAM OF HOT GAS WHICH ATTENUATES THE RESIN INTO FIBERS HAVING DIAMETERS BETWEEN ABOUT 10-40 MICRONS. THE FIBERS ARE COLLECTED ON A CONTINUOUSLY MOVING SURFACE POSITIONED FROM ABOUT 10 TO ABOUT 30 INCHES FROM THE DIE OPENINGS.

Aug. 28, 1973 .1. P. KELLER ET AL 3,755,527

PROCESS FOR PRODUCING MELT-BLOWN NONWOVEN SYNTHETIC POLYMER MAT HAVING HIGH TEAR RESISTANCE Filed Oct. 9, 1969 DIE HEAD 8 EXTRUQER 3 FIBERS INVENTORS.

JAMES P. KELLER, JAMES s. PRENTICE, BY JOHN w. HARDING,

Ewes;

ATTORNEY.

United States Patent 3,755,527 PROCESS FOR PRODUCING MELT-BLOWN NON- WOVEN SYNTHETIC POLYMER MAT HAVING HIGH TEAR RESISTANCE James P. Keller, James S. Prentice, and John W. Harding,

Baytown, Tex., assignors to Esso Research and Engineering Company Filed Oct. 9, 1969, Ser. No. 865,105 Int. Cl. D01d /12 US. Cl. 264210 F 14 Claims ABSTRACT OF THE DISCLOSURE A highly tear resistant nonwoven mat of thermoplastic polymer fibers is made in a melt-blowing process in which a molten polymeric resin is extruded through a row of die openings into a stream of hot gas which attenuates the resin into fibers having diameters between about -40 microns. The fibers are collected on a continuously moving surface positioned from about 10 to about 30 inches from the die openings.

BACKGROUND OF THE INVENTION (1) Field of the invention The present invention is directed to a melt-blowing process for producing highly tear resistant nonwoven mats composed of thermoplastic polymer fibers which have diameters from about 10 to about 40 microns.

(2) Description of the prior art SUM MARY OF THE INVENTION In this invention, a thermoplastic polymeric resin is extruded in molten form through a row of die openings in a die head into a converging stream of hot gas emerging from gas slots immediately above and below the row of die openings. The hot gas is moved at rates, relative to the rate of polymer flow, which attenuate the polymer into fibers having a diameter from about 10 to about 40 microns. The fibers, which are attenuated essentially in a plane away from the die openings, are collected as a non-woven mat on a continuously moving surface positioned from about 10 to about 30 inches or more from the die openings of the die head. In a specific aspect of the invention in which the fibers are polypropylene fibers, the melt blown nonwoven mat has a tear resistance of at least about 1000 dm. and a strip tensile strength of no more than about 800 m.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic perspective view of the overall melt-blowing process; and

FIG. 2 is a detailed view in longitudinal cross section of a die which may be used in the melt-blowing process.

Patented Aug. 28, 1973 ice Referring to FIG. 1 of the drawings, a thermoplastic polymer is introduced into a pellet hopper 1 of an extruder 2. The thermoplastic polymer is forced through the extruder 2 into a die head 3 by a drive 4. The die head 3 may contain heating means 5 which may control the temperature in the die head 3. The thermoplastic polymer is then forced out of a row of die openings 6 in the die head 3 into a gas stream which attenuates the thermoplastic polymer into fibers 7 which are collected on a moving collecting device 8 such as a drum 9 to form a continuous mat 10. The gas stream which attenuates the thermoplastic polymer is supplied through gas jets 11 and 12 respectively, which are more clearly seen in FIG. 2. The gas slots 11 and 12 are supplied with a hot gas, preferably air, by gas lines 13 and 14 respectively.

The process may be further understood by reference to the details of the die head 3, as more fully depicted in FIG. 2. The die head 3 is formed of upper die plate 15 and lower die plate 16. The thermoplastic polymer is introduced in the back of the die plates 15 and 16 through an inlet 17 as a result of the forcing action of extruder 2 at the back of the die plate 3. The thermoplastic polymer then goes into a chamber 18 between the upper and lower die plates 15 and 16 respectively. The facing of the die plate 16 may have milled grooves 19 which terminate in the die openings 6. It is understood, of course, that the milled grooves may be in the lower die plate 16 or the upper die plate 15, or that grooves may be milled in both plates 15 and 16. Still further, if a single plate is used in place of the upper and lower die plates, the grooves may be drilled to produce the die openings 6. An upper gas cover plate 20 and a lower gas cover plate 21 are connected to the upper die plate and lower die plate 15 and 16 respectively to provide an upper air chamber 22 and a lower air chamber 23 which terminate in the gas slots 11 and 12 respectively. The hot gas is supplied through inlet 24 and upper gas cover plate 20 and inlet 25 and lower gas cover plate 21. Suitable baffling means (not shown) may be provided in both the upper air chamber 22 and the lower air chamber 23 to provide a uniform fiow of air through the gas slots 11 and 12 respectively. The die head 3 may contain heating means 5 for heating both the thermoplastic polymer and air in the die head 3.

The particular operating conditions employed in the melt-blowing process will control the characteristics of the nonwoven thermoplastic polymer mats produced by that process.

In accordance with this invention, it has been discovered that melt-blowing operations can be made to produce fibers of diameters from about 10 to about 40 microns and that nonwoven thermoplastic polymer mats of high tear resistance are made if those fibers are collected at a distance greater than 10 inches from the die openings.

In operating the melt-blowing process to produce fibers having diameters between about 10-40 microns, smooth molten flow of the polymeric resin of choice and smooth attenuation of the fibers is required. This is achieved through the selection and control of the appropriate combination of die tip temperature, resin flow rate, and resin molecular weight to give an apparent viscosity in the die holesof from about to about 800 poise, preferably within the range of from about 50 to about 300 poise. For a particular resin, by measuring the pressure upstream of the die holes and by measurin the polymer flow rate, the apparent viscosity is calculated from the geometry of the die by methods well known in polymer rheology. See, e.g. both H. V. Boenig, Polyolefins, p. 264 (1966) and Chemical Engineering Handbook (Perry ed. 1950) at p. 375. The viscosity can usually be adjusted into the operable range by varying the die tip temperature.

Herein, polypropylene resin is used to illustrate the present invention. Other thermoplastic polymeric resins suitable for such use include other polyolefins, e.g., polyethylene; polyesters, e.g., poly(methylmethacrylate) and poly(ethyleneterephthalate); polyamides, e.g., poly(hexamethylene adipamide), poly(w-caproamide) and poly- (hexamethylene sebacamide); polyvinyls such as polystyrene; and other polymers such as polytrifluorochloroethylene.

To be melt blown into fibers, polypropylene, it has been found, must be thermally treated at temperatures in excess of 550 F., up to about 900 F. and preferably, within the range of from about 550 to about 800 F. The degree of thermal treatment necessary varies with the melt index of the particular polypropylene resin employed and with the rates used in the melt blowing process. The thermal treatment may be carried out in the extruder 2 alone or partially in the extruder and partially in the die head 6.

The polymer flow rate, the rate at which the polymer is forced through the die openings 6 in the die head 3, is dependent upon the specific design of the die head and extruder. However, suitable polymer flow rates are from about 0.07 to about 0.5 or more gm./min./opening. The polymer flow rate may be controlled by the speed of the extruder. The gas flow rates are also limited by the design of the die head. Suitable products have been obtained at air rates from about 0.2 to about 4 lbs/min.

The fiber diameters of the nonwoven mats of this in vention are achieved by adjusting the gas flow rates for a given molten polymer flow rate so that one obtains a pounds of gas/pounds of polymer ratio of from about 10 to about 60, preferably, between about 25 and about 50. Air rates of this magnitude serve to attenuate the molten resin extruded through the die openings into fibers having diameters from about 10 to about 40 microns, usually from about to about 25 microns. When the air rates for a given polymer flow rate are too low, large coarse fibers are formed which entwine into coarse, ropey bundles or rope that produces coarse, nonpliable, brittle, irregular mat structure. Then, as air flow rates are increased and pass though the range which provides the rope free, highly tear resistant nonwoven mats of this invention, so that the air rates are too high relative to the rate of polymer flow, the attenuated fibers break and become discontinuous and produce large objectionable shot in the nonwoven mat. The shot may be as large as 1 millimeter in diameter. If the mat is calendered, this type of shot appears as a large clear area in the mat, giving the mat a coarsely mottled appearance. At even higher air rates, relative to the polymer flow rate, sho gets much smaller and nonwoven mats composed of very fine fibers from about 1 to about 10 microns are formed having very poor tear resistance.

Aside from the rate of air flow relative to the rate of molten polymer flow, the single most important factor in producing the highly tear resistant nonwoven mat of this invention is the distance separating the collecting device 8 from the die openings 6 in the die head 3. Unless the contained heat of the deposited fibers is quenched by auxiliary cooling media applied to the collecting device 8, it is necessary to space the collecting device or collector at least about 10 inches from the die openings so that the fibers will dissipate most of their heat before deposition on the collector. At die-collector-distances of 10 or more inches, the fibers in nonwoven mats of the present invention are bound together essentially by entanglement, with little or no self-bonding, any self-bondil'lg decreasing with increasing die-collector-distances. The term self-bonding herein means thermal bonding of one fiber to another as the nonwoven thermoplastic polymer mats are formed. Advantageously, the die-collectordistance is no greater than about 30 inches, and desirably it is from about 12 to about 24 inches, preferably, from about 18 to about 24 inches.

Polypropylene nonwoven mats which are prepared at die-collector-distances of from about 10 to about 30 inches under melt blowing conditions producing fiber diameters of 10-40 microns have tear resistances in excess of 1000 dm. as measured by a standard Elmendorf tear strength tester in accordance with ASTM procedure D-689-62. The tear resistance of the preferred mats is greater than 2000 dm. and may be 3000 dmfi or more. Tear resistance is reported in units of dmF, the result of dividing the average force necessary to tear the mat, in grams, by the basis weight of the mat, in grams per square meter, all of which is multiplied by 100. The strip tensile strength of these highly tear resistant nonwoven mats has rarely been found to exceed 700 m., as measured by ASTM procedure D-828-60 using a standard Instron tester with a two-inch jaw separation and an elongation rate of 250 percent per minute. The results of the strip tensile strength measurements are reported in meters, the unit resulting when the drawing necessary to break the mat, measured in grams, is divided by the width of the sample, measured in meters, and all of which is divided by the basis weight of the sample in grams per square meter. The nonwoven mats can be drawn in this test from about 200 to about 500 percent before breaking, evidence that very drawable fibers are formed by the melt-blowing conditions used to make the mats.

The nonwoven mats may be compacted at room temperature to densify them, if desired. Calender roll devices are suitable for this purpose.

The present invention is further illustrated by the following specific examples which are given by way of illustration only, and not as limitations on the scope of the invention.

EXAMPLES 1-7 The highly tear resistant nonwoven mats of the present invention are illustrated by Examples 1-7 presented in Table I hereinafter. The nonwoven mats of Examples l-7 were made by the melt-blowing process which was illustrated in FIGS. 1 and 2 of the drawings, using the specific operating conditions set forth in Table I. The melt-blowing process of Example 1 employed a 4 inch long, hole die head; that of Examples 2-7 used a 10 inch long, 200 hole die head. The melt blown mats were compacted prior to testing by calendering them at room temperature under a calender roll pressure of 500 pounds per linear inch and at a line speed of 20 feet per minute. The tear resistances, strip tensile strengths and percent elongations, and basis weights of the nonwoven mats which resulted are shown in Table I. The fibers of the mats had diameters within the range from about 10 to about 40 microns, usually from about 18 to about 25 microns.

TABLE I Example number 1 2 3 4 5 6 7 3a 33 33 a3 a3 a3 a3 iiiriiiiiiiii-iii 252 2a a? 222 2a 222 a s5 20 21 21 21 21 21 0.82 1.4 1.4 1.4 1.4 0.9 0.9 44 32 so :10 a0 20 20 12 1s 1s 18 1s 1s 1s 1,304 2,051 1,998 3,005 1,915 2,381 584 594 588 9 422 555 230 250 425 525 450 Basis wt. (gmJmJ) 74 81 4s 66 34 EXAMPLE 8 2. The improved process of claim 1 in which said This example illustrates the diiferent kind of mat ch'argg i 1 f ggf if wthm g i acteristics which occur when the melt-blowing conditions on a on Perm i effective to cause said thermal degradation of said resin are such as to produce fibers smaller than the present H h range of from about 10 to about 40 microns. The die-colu rfasm asdsal 1 2 h lector-distance is 12 inches in both cases. e linprove q calm w erem 20 thermoplastic polymer resin 1s selected from polyamides, TABLE H polyesters, polystyrenes and polyolefins.

4. The improved process of claim 2 wherein said ther- Example number 8 1 moplastic polymer resin is polypropylene. Polypropylene MFR 0.6 33 5. The improved process of claim 1 wherein said 322 3??? 333 22g 25 resin is subjected to said thermal degradation at least Polymer r ate (glil iz til li3t 8.4 8.5 partially in an extruder feeding said resin into said nozzle. f 7 g;- 5 '22 6. The improved process of claim 1 wherein said Die 55115518: digstan e t. 12 12 gas flow rate is from about 0.2 to about 4 pounds per NODWOVQI! 1 118 proper 185! I i t Tear mslstance (dm'z) 7. The improved process of claim 1 wherein said flow Basis wt (gm/mo I 53 rate is effective to attenuate said resin into fibers having diameters from about 15 to about 25 microns.

8. The improved process of claim 1 wherein said Clearly, the nonwoven mat of fibers of smaller diameter pounds f gas to pounds of polymer ratio is from about than 10 microns has a tear resistance very much lower 25 to about 5 than the tear resistance of the nonwoven mat prepared ac- 9 The improved process of claim 1 wherein said cording to th Present inventiondistance of said receiver from said nozzle orifices is from The high tear resistances of the present nonwoven mats about 10 to about 30 inches makes them well suited for such applications as wrapping The improved process of claim 1 wherein said ma e Clothing liners, diaper liners, components In distance of said receiver from said nozzle orifices is laminates, and like articles requiringhigh tear strength. f about 12 to about 24 inches Having fully and Particularly descnbed i hlgilly tear 11. The improved process of claim 1 in which said resistant melt-blown nonwoven mats of this invention, and distance of Said receiver from Said nozzle orifices is the method of producing such mats, it will be understood from about 18 to about 24 inches that Variations and modificatioqs can be made Within the 12. In a process for producing a melt-blown nonwoven spirit and scope of the invention as defined 1n the apmat wherein a fib f i polypropylene resin is pended f truded in molten form through orifices of a heated nozzle We clalmi into a stream of hot gas flowing in the same direction In a ProcesS for profjucmg a meltmown as said molten resin to attenuate said molten resin as mat wherein a fiberformmg thermoplasilc Polymer T651 fibers in a fiber stream, said fibers being collected on a is extruded in molten form through orifices of a heated receiver in the path of said fiber stream to form Said nozzle into a stream of hot gas to attenuate said molten nonwoven mat, resin as fibers in a fiber stream, said fibers being collected the improvement of producing a me1t .b1own now on a receiver in the path of said fiber stream to produce Woven polypropylene mat having a tear resistance sald nqnwoven of greater than 1000 dmfi, and a strip tensile strength the improvement of produc ng a melt-blown nonwoven of less than 800 m" comprising:

mat having a reslstance greater than 1000 subjecting said resin, prior to extrusion thereof into "i a tenslle strength of less than 800 said stream of hot gas, to a temperature within the P P f range from about 550 F. to about 900 F. for a sublecmig sald resin to .ther.ma1 defgdradanon period of time eifective to thermally degrade said extr'uslqn Said. F Into Sal Stream resin until said resin has a viscosity in said nozzle gas until said resin 1s degraded to have a viscosity f fr b t 50 t b t 300 d in said nozzle orifices of about 50 to about 300 on 0 o a Polse unng poise during extrusion therethrough, extgilslon g thr h f extruding said degraded resin into said stream of hot extm mg f e (mg a row 0 gas at a resin flow rate of from about 0.07 to about nofzzle onfices Into sald stream of hot gas at a 05 gram/minute/orifice. resin flow rate of from about 0.07 to about 0.5 gram/ flowing said stream of hot gas at a rate of from about F 9 10 to about 60 pounds of gas per pound of polymer flowing Sald stream hot f Slots effective to attenuate said resin into fibers having medlately above a below of Said TOW p g diameters from about 10 to about 40 microns, and i at f p F0 p -p y fl fate collecting said fibers on said receiver at a distance Tail0 of from about 10 about effectlve to from said nozzle orifices efiective to c u id tenuate said molten resin into polypropylene fibers fibers to be bound together essentially completely having a diameter of from about 10 to about 40 by self-entanglement. microns, and

collecting said polypropylene fibers on said receiver at a. distance from said nozzle orifices of from about 10 to about 30 inches.

13. The improved process of claim 12 wherein said pounds of gas to pounds of polymer ratio is from about 25 to about 50.

14. The improved process of claim 13 wherein said distance from said nozzle orifices to said receiver is from about 12 to about 24 inches.

References Cited UNITED STATES PATENTS 8 4/1968 Hartmann et a1. 264210 F 3/1970 Hartmann et a1. 264210 F 12/1961 Maragliano et a1. 264211 8/1964 Roberts et a1. 264211 3/1969 Papps 264210 F 4/1969 Siggel et a1. 264171 12/1970 Wagner et a1. 264210 F 8/1962 Roberts et a1. 264210 F 2/1966 Benson 264168 12/1971 Polestak et a1. 264210 F OTHER REFERENCES Superfine Thermoplastic Fibers," by Wente, Ind. Eng. Chemistry, 48 (8): 1342-1346 (August 1956).

JAY H. WOO, Primary Examiner US. Cl. X.R. 

